FIG. 1 generally illustrates a turning tool 1 during conventional, external machining of a workpiece 2. The tool 1 includes a holder 3, as well as, a replaceable turning insert 4 made in accordance with the present disclosure. In this case, the workpiece 2 is rotated (in the direction of rotation R) at the same time as the tool is longitudinally fed parallel to the center axis C1 of the workpiece 2, more precisely in the direction of the arrow F. The longitudinal feed per revolution is designated f, while the cutting depth is designated ap. The entering angle between the direction of the longitudinal feed and a main edge included in the turning insert is designated κ. In the example shown, κ amounts to 95°.
It should furthermore be pointed out that the turning insert 4 has a rhombic basic shape and includes two acute corners having an angle of 80° and two obtuse corners having an angle of 100°. In such a way, a tool back clearance σ of 5° between the turning insert and the generated surface of the workpiece is obtained. Usually, the holder 3 is manufactured from steel and the turning insert 4 from cemented carbide or the like.
In all kinds of chip removing machining of metal, including turning, the rule applies that the chip “is born crooked”, i.e., immediately after the moment of removal, the chip obtains an inherent aim to be curved. The shape of the chip, among other things its radius of curvature, is determined by several factors, the most important of which, in connection with turning, are the feeding of the tool, the rake angle of the cutting edge, the cutting depth in question, as well as the material of the workpiece. After the removal, the chip will move perpendicular to each infinitesimal part of the cutting edge. If the cutting edge is straight, the chip therefore becomes flat or cross-sectionally rectangular, but if the same is entirely or partly arched, the chip becomes cross-sectionally entirely or partly arched.
Another factor, which has bearing on the turning process, is the choice of the so-called cutting geometry of the cutting edges. Two categories of cutting edges are distinguished by those skilled in the art, on one hand, cutting edges having a positive (nominal) cutting geometry, and, on the other hand, cutting edges having a negative cutting geometry. In the first-mentioned case, a wedge angle of the cutting edge, i.e., the angle between the chip surface and the clearance surface, which together form the cutting edge, is smaller than 90° (=acute), while the wedge angle of the cutting edge in the second case amounts to 90° (or more). A crucial difference between a cutting edge having a positive cutting geometry and one having a negative is that the first-mentioned one lifts out the chip by being wedged in between the same and the generated surface, while the last-mentioned one pushes the chip in front of itself while shearing off the same. Therefore, positive cutting edges generally become more easy-cutting than negative ones, and produce chips having greater radii of curvature than chips produced by the last-mentioned ones.
In order to provide additional background of the nature of the chip produced in connection with turning, attention is directed to a metaphor, which is used by those skilled in the art to explain the fact that chips having different width/thickness have different bendability. Thus, a thin and narrow chip may be compared to a slender blade of grass, while a thick chip may be compared to a stiff reed. Similar to the blade of grass, a thin chip can without appreciable difficulty be bent if the same is brought toward an obstacle in the form of an adjacent, more or less steeply sloping guide surface, while a stiff, reed-like chip would be over broken under the same conditions; this is something that causes a high sound level, great cutting forces, a short service life of the turning insert, as well as a high generation of heat, possibly accompanied by sticking.
In connection with turning, the chip control is of large importance, not only to the machining result, but also to an efficient, problem-free operation. If the removed chip would not be guided by any guide surface or chip breaker, the same will be developed in an uncontrolled and unforeseeable way. Thus, at least thin and bendable chips may curl in long, telephone cord-like screw formations, which may impinge on and damage the generated surface of the workpiece, and—not the least—get entangled in the tool or other components included in the machine in which machining takes place. If a thick and stiff chip, on the other hand, shortly after the removal would impinge on a steeply sloping guide surface, other problems will arise, such as tendency to over breaking of the chip, extreme generation of heat, which may entail sticking, and that the turning insert becomes blunt-cutting, as well as risk of premature wearing damage in the guide surface. Therefore, an optimum, desirable chip control is obtained if the guide surface of the chip-former is situated at such a distance from the cutting edge line of the cutting edge, and at such an angle of inclination that the chip is carefully guided away, in such a way that the same can be broken into smaller fragments, e.g. by being curled or brought to impinge on the clearance surface of the turning insert and being broken into pieces against the same. Even if helicoidal chips, rather than short fragments, would be formed, it is desirable that the same have a small diameter and a limited length.
In this connection, it should be pointed out that good chip control is particularly important in modern, software-controlled turning or multi-operation machines, which are placed in sealable housings and periodically unmanned. If the chips are not divided into smaller fragments (or short screw formations), which can be carried away via a conveyor included in the machine, but rather form tangles of helicoidal chips, such may cause shutdown and serious damage to the machine.
Within the field of turning, it is desirable to be able to use one and the same turning insert for roughing, medium, and finishing operations while attaining good chip control irrespective of the cutting depth in question. For this reason, a number of different turning inserts have been developed having chip-formers, which include, on one hand, a breast surface placed behind the individual nose edge to guide such narrow chips that are formed when the cutting depth is small (finishing), and, on the other hand, two flank surfaces placed inside the chip surfaces of the main edges and having the purpose of guiding such narrow chips that are wide as a consequence of the cutting depth being large (roughing). Examples of such turning inserts are documented in U.S. Pat. No. 5,372,463, U.S. Pat. No. 5,743,681, and U.S. Pat. No. 7,374,372.
In spite of all the development attempts, the turning inserts in question have, however, a mediocre versatility with respect to the ability to guarantee good chip control under all the varying operation conditions that occur in practice. Thus, certain turning inserts may give acceptable results when the cutting depth is small and the feed moderate (=narrow and thin chip), but poor results when the cutting depth as well as the feed are increased (=wider and thicker chip). This lack of versatility becomes particularly annoying when the cutting depth varies during one and the same working operation.
In order to remedy the shortcomings of the previously known techniques, a turning insert has been developed, which is the subject of SE 1150869-4 (filed on 2011-09-23). Characteristic of this turning insert is that the same includes a knob placed closely behind the nose edge of the cutting edge and having a breast surface, and a geometrical shape described further herein. Briefly, the breast surface of the knob may be said to have a convexly arched shape and be elongate and transverse in relation to a bisector between the main edges, as well as have an inclination that is the greatest in the middle so as to then successively diminish toward those ends that are situated closest to the main edges.
Although this turning insert has given good results in many different applications, it has turned out that the chip guiding under certain conditions, for example, when the cutting depth is small (=narrow chips) and the feed large (=relatively thicker chips), has not been satisfying. Thus, such chips (above all from materials difficult to machine) have been able to pass or “jump over” the breast surface without being affected by the same. This means that the chips will not be fragmented but developed in an uncontrolled way.